In the field of LED drivers for offline applications such as retrofit lamps, solutions are demanded to cope with efficiency, high power density, long lifetime, high power factor and low cost among other relevant features. While practically all existing solutions comprise one or another requirement, it is essential that the proposed driver circuits properly condition the form of the mains energy to the form required by the LEDs while remaining in compliance with present and future power mains regulations. In addition, it is required that the driver circuits comply with existing power adjustments, e.g. dimmers or the like, so that the drivers can be used universally as a retrofit driver device including the LED units.
Dimmable LED retrofit lamps need to be compatible with a wide range of existing dimmers. Most of those dimmers are designed for operation with incandescent light bulbs. However, the input characteristics of LED retrofit lamps can be quite different from those of incandescent light bulbs. Therefore, special driver devices are required for correct operation of the dimmers and the LED lamps.
The driver circuits should comply with all kinds of dimmers, especially phase-cut dimmers, which are preferably used to regulate the mains voltage with low power loss. Those dimmers are usually used to regulate the mains energy provided to an incandescent light bulb which needs a low load impedance path for a timing circuit operating current to adjust the phase-cut timing. The provision of this low load impedance path has to be adjusted to the zero crossing of the mains voltage, in particular at low power operation of the LEDs. In particular, during low power operation, a high impedance path has to be provided before the zero crossing and the low impedance path has to be provided after the zero crossing.
EP 2 282 608 A2 discloses a light apparatus comprising an LED assembly including a current sensor to detect the zero crossing of the supply voltage. The current sensor comprises a plurality of measurement resistors which detect the current provided by the power supply to the LED units. This current measurement unit influences the current provided to the LEDs and reduces the power factor due to the high power loss within the measurement resistors.